An integrated circuit requires conductive interconnects between semiconducting domains in order to communicate signals therebetween. In order to create ever faster microprocessors, smaller dimension interconnects of higher conductivity materials is an ongoing goal.
As microelectronic efficiencies have increased, interconnects have decreased in dimensional size and efforts have been made to increase the electrical conductivity of interconnect features. It is becoming increasingly difficult to design and fabricate ultralarge scale integrated circuits (ULSI) chips with the desired high packing density and high-speed operation. This is not due to the difficulty of transistor scaling but to the interconnect delay in distributing high frequency signals across the ULSI chip. Therefore, the significance of interconnect technologies has become much greater in view of the ongoing need for ever smaller interconnects. Accordingly, there is a need for novel interconnect-forming technologies.
The rapid miniaturization of interconnects is occurring simultaneously with the transition from aluminum (Al) to copper (Cu) metallization for sub-0.25 μm interconnects (IC). It is now becoming apparent that a major component of improved interconnect performance will consist in replacing aluminum, the previous metal of choice, with copper. Murakami et al., J. Vac. Sci. Technol. B. 1999, 17, 2321-2324, have indicated that due to its electromagnetic resistance, low resistivity, and high reliability against electromigration Cu is thought to be the most attractive substitute for Al alloys in integrated circuit manufacturing. The transition from Al to Cu has led to a change in the way interconnects are formed. While Al has been deposited as a blanket layer which is then patterned by reactive ion etching, Cu interconnects are formed by evaporative deposition into preformed (damascene) trenches and vias followed by chemical mechanical polishing (CMP).
As the interconnect width decreases and the aspect ratio increases, conventional vacuum deposition techniques approach the theoretical resolution threshold. Deep, narrow trenches and vias preferentially collect material at the damascene feature edges, leading to void formation. Blanket and selective chemical vapor deposition (CVD) are well-established Cu deposition techniques that have a demonstrated ability to fill current interconnect trenches. (A. E. Kaloyeros and M. A. Fury, MRS Bull. (June 1993), pp. 22-29). This process of involves heating a metal until it vaporizes and then condensing the vaporized metal condenses onto a cold surface. The process is cumbersome in terms of time, money and energy. Additionally, heating of the IC substrate during CVD to assure crystalline growth degrades fine architecture structures on the substrate.
The cost of Physical vapor deposition (PVD) and chemical vapor deposition (CVD) equipment capable of performing either of these processes is about $1 million. This cost does not include the cost, time, and materials associated with intermediate polishing prior to fill. Additionally, PVD is a competing technique to CVD. The formation of a seed layer is essential prior to trench filling by electrochemical deposition (ECD). ECD may be used to achieve conformal fill of ICs, trenches, and vias into which a seed layer has been grown by CVD or PVD. Thus, existing methods require additional steps of (a) depositing seed layers prior to fill, (b) intermediate cleaning between seed layer deposition, and (c) a final chemical mechanical polishing step to remove the conformal metal coating after fill. In addition to the high purchase cost of a separate chamber to perform each of these additional steps, each step adds about $1 per metallization layer in consumable materials cost.
In any case, CVD does not inherently fill trenches preferentially over any other portion of substrate having nucleation sites. Therefore a method that preferentially deposits Cu into trenches based on differential solvent evaporation associated with trenches is needed.
Thus, the semiconductor industry is in need of a copper interconnect formation process capable of achieving higher resolution at lower temperature and ideally, at a lower cost. The successful synthesis of Cu nanocrystals will offer the semiconductor industry a thermodynamically metastable source for copper metallization. Based on the literature and reagent costs, copper nanocrystal synthesis could be expected to yield a suitable precursor at a lower cost than CVD.
The mesoscopic size regime between atoms and bulk materials is characterized by unusual properties. Mesoscopic systems exhibit collective atomic behavior, but not to a sufficient extent so as to preclude quantized effects. Many of the unusual thermodynamic and spectroscopic anomalies associated with mesoscopic systems are attributable to surface effects. Studies have shown surface energies that are 10 to 400% greater for nanocrystals than for bulk gold (Au) and platinum (Pt) (C. Solliard and M. Flueli, Surf. Sci. 156 (1985), pp. 487-494), and Al (J. Wolterdorf, A. S. Nepijko and E. Pippel, Surf. Sci. 106 (1981), pp. 64-72). In the bulk, surface atoms represent such a small percentage of the total that surface effects are largely inconsequential. Surfaces generally possess modified atomic coordination numbers, geometries and diminished lattice energies relative to the bulk. The result of these modifications is that physical, spectroscopic, and thermodynamic properties, which are constant in the bulk, become size dependent variables in nanocrystals. The ability to modify the thermodynamic properties of nanocrystals, particularly the melting temperature, is exploited in the present invention to produce thin film copper IC structures at low temperature.
Metallic nanocrystals have been shown to reduce melting temperatures compared with the bulk. (Ph. Buffat and J-P. Borel, Phys. Rev. A, 13 (1976), pp. 2287-2298; C. J. Coombes, J. Phys. 2 (1972), pp. 441-449; J. Eckert, J. C. Holzer, C. C. Ahn, Z. Fu and W. L. Johnson, Nanostruct. Matls. 2 (1993), pp. 407-413; C. R. M. Wronski, Brit. J. Appl. Phys. 18 (1967), pp. 1731-1737 and M. Wautelet, J Phys. D, 24 (1991), pp. 343-346). The depression in melting and annealing temperature is evident throughout the nanocrystal size regime, with the most dramatic effects observed in nanocrystals having a diameter from 2 to 6 nm. Melting studies on a range of nanocrystals have established that the melting temperature is size dependent in the nanometer size regime and is approximately proportional to the inverse particle radius regardless of the material identity. The size dependent melting temperature of metallic nanocrystals has included studies of Au, Pb and In, Al and Sn. (Au: Ph. Buffat and J-P. Borel, Phys. Rev. A, 13 (1976), pp. 2287-2298; Pb and In: C. J. Coombes, J. Phys. 2 (1972), pp. 441-449; Al: J. Eckert, J. C. Holzer, C. C. Ahn, Z. Fu and W. L. Johnson, Nanostruct. Matis 2 (1993), pp. 407-413; and Sn: C. R. M. Wronski, Brit. J. Appl. Phys. 18 (1967), pp. 1731-1737).
The reduction in melting temperature as a function of nanocrystal size can be enormous. For example, 2 nm Au nanocrystals are predicted to melt at about 300 degrees Celsius, as compared to 1065 degrees Celsius for bulk gold. (M. Wautelet, J. Phys. D, 24 (1991), pp. 343-346).
Numerous techniques exist for the preparation of metal colloids. Surprisingly, little attention has been paid to monodispersity and solubility of Cu nanocrystals. Monodispersity refers to a unimodality or uniformity of nanocrystal size in solution as opposed to polydispersity which connotes a solution having a range of nanocrystal sizes. Monodisperse Au nanocrystals having an alkane-thiol capped surface are readily synthesized in a two-phase reaction, with resulting particles being highly soluble in toluene and/or hexane. Many liquid phase syntheses also exist for Ag nanocrystals, but with less control over particle size distribution. Silver nanoparticle syntheses are only to a very limited extent adapable to making Cu nanocrystals. Inherent inadequacies in conventional nanoparticle synthesis technology with respect to polydispersity and solubility, preclude the formation of high conductivity interconnects necessary for internal communication within an IC. For example, the polydispersity of Cu nanocrystals made by conventional methodology, results in a broad range of melting temperatures and leads to comparatively high resitivity in interfacial regions between particles melting at variable temperatures.
Unsuccessful attempts have been made to obtain Cu nanocrystals involving a number of methodologies. INn one method hydrazine carboxylate was used as a ligand for Cu2+. The intent was to produce copper nanocrystals from the decomposition of the hydrazine carboxylate complex of Cu2+ in the presence of a passivating agent. Hydazine carboxylate would act as a reducing agent for copper, itself being oxidized to gaseous products. This experiment failed to produce any copper nanocrystals. Problems included but are not limited to: poor solubility of the copper hydrazine carboxalate complex and recovery of only bulk copper from the reaction mixtures. Additionally, hydrazine carboxylate and complexes thereof are potentially explosive making them diffuclt to work with. As such, this method is not capable of forming suitable nanocrystal colloids.
A proposed alternative to make Cu nanocrystals involved reducing Cu salts in water and pyridine, followed by the addition of sulfur-containing ligands, such as alkylthiols, alkylthiophenes, and alkylxanthates initially showed great promise. They afforded nanocrystals that generally possessed good solubility in a wide range of organic solvents. Air-stability of nanocrystals passivated by the above ligands was found to be excellent. Copper nanocrystals passivated by the above mentioned sulfur-containing ligands were characterized by Transmission Electron Microscopy(TEM), Powder X-ray Diffraction (XRD), Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Atomic Force Microscopy (AFM). Data generally agreed with that expected for copper nanocrystals having a size between 2 nm and 10 nm. However, the sulfur-containing set of passivating agents/ligands was ultimately abandoned when it became apparent that nanocrystals passivated by such ligands led to Cu2S or Cu2S/Cu mixtures under thermolysis conditions. Generally thermolysis under an inert atmosphere (nitrogen or argon) gave Cu2S, whereas thermolysis under a reducing atmosphere (5% Hydrogen/95% nitrogen). Thermolysis products were determined by XRD.
Finally, syntheses using alkylamine-ligands and reduction in the presence of pyridine, also yielded copper nanocrystals. However, such syntheses generally yielded a black, insoluble powder (>95%) and a very faintly colored amber solution. The solution was found to contain copper nanocrystals in the range of 2 nm to 10 nm in size by AFM. The amount of soluble material present was certainly so miniscule (probably microgram scale) that any application of such nanocrystals would be impossible.